Uniform electrostatic charging



United States Patent lnventors Walter Roth Rochester, N.Y.; Alex E. Jvirblis, Santa Clara, Calif. Appl. No. 676,278 Filed Oct. 18,1967 Patented Dec. 22, 1970 Assignee Xerox Corporation Rochester, N .Y. a corporation of New York UNIFORM ELECTROSTATIC CHARGING 5 Claims, 5 Drawing Figs.

US. Cl. 317/262, 250/495, 96/1, 317/3 Int. Cl G03g 13/00 Field of Search 317/3, 4, 262; 250/495; 355/3, 17; 96/1 Primary Examiner-J. D. Miller Assistant Examiner-William J. Smith Attorneys-Ronald Zibelli, Paul M. Enlow, James J. Ralabate and Norman E. Schrader ABSTRACT: This invention relates to a method for uniformly charging insulating surfaces comprising providing a surface having a nonuniform charge density thereon and thereafter contacting said nonuniformly charged surface with a material causing the charged surface to attain a uniform charge density in contacted areas.

PATENTED 050221970 8549862 Ill Ill II FIG. IA

l/ FIG. 16

F/ 6. ID

INVENTOR. ALEX E. JVIRBLIS v, ALTE R0 f 1 UNIFORM ELECTROSTATIC CHARGING BACKGROUND OF THE INVENTION This invention relates to a method for electrostatically charging insulating surfaces and, more particularly, to a method for charging insulating surfaces to a substantially uniform negative potential. More specif cally, the invention relates to a method for uniformly redistributing a nonuniform negative corona charge on an insulator-or photoconductive insulator surface. 1 I I g In the process of xerography, for example, as disclosed in Carlson U.S. Pat. No. 2,297,65 l a xerographic plate comprising a layer of photoconductive insulating material on a conductive backing is given a uniform electric'charge over its surface and is then exposed to the subject matter to be reproduced, usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the radiation intensity that reaches them, and thereby creates an electrostatic latent image onor' in .the photoconductive layer. Development of the latent image is effected with an electrostatically charged, finely divided material, such as an electroscopic powder, that is brought into surface contact with the photoconductive layer, the powder depositing in a pattern corresponding to the latent electrostatic image. Thereafter, the developed xerographic powder image is usually transferred to a support. surface to which it may be fixed by any suitable means.

In the practice of the xerographic'process as described in the preceding paragraph, there has been a need for charging the photoconductive insulating layer to a uniform potential on the order of several hundred volts. .It has been usual to charge the plate with corona of positive polarity by means of a corona generating device, which when supplied :with potential above the corona threshold produces anerni'ssion of corona ions.

Typical embodiments of corona'generating devices are disclosed in the Walkup U.S. Pat. No. 2,777,957 and in Vyverberg U.S. Pat. No. 2,836,725. Apparatus of the former is referred to in the-art as a scorotron' and includes a plurality of high voltage corona discharge electrodes combined with a wire screen for regulating corona emission. Apparatus of the latter is referred to in the art as a corotron which utilizes only a single corona wire.

' As indicated above; it is customary when charging a xerographic plate, especially a 'selehium xerographic plate, to apply corona of positive polarity. This is because it has been found that while selenium can conductboth electrons and holes the mobility for holes is approximately 10 times that for the electrons. Accordingly, negative corona charging and dissipation thereof in response to an optical pattern is normally insufficient to properly discharge the photoconductive layer.

Rather, the electrons are trapped within the bulk of the photoconductive layer establishing a space charge which prevents effective reproduction of the original image.

Xerographic plates have been. developed, on the other hand, which are capable of accepting-corona of negative polarity and dissipating such chargein response to an optical pattern. Sticliplateshave a sufficiently high mobility for electrons to substantially dissipate the charge in illuminated areas. Exemplary plates are those disclosed by Greig U.S. Pat. No. 3,052,539 and by Middleton et al. U.S. Pat. No. 3,121,006.

It has been found, however, that use of the conventional corona charging devices for negative charging results in a nonuniform charge distribution which is not totally acceptable for xerographic purposes. Nor is it totally acceptable where imaging takes place on an insulator, as opposed to a photoconductive insulator, surface. Negative corona has a tendency to concentrate itself at discrete points. along the corona wire with the result that negative charge deposits on the insulating layer in a nonuniform pattern corresponding to the nonuniforrnity of the corona discharge itself, while positive corona discharge from a wire generally occurs in a volume in the formof a continuous uniform sheath surrounding the wire. Designs correcting for this deficiency provide complicated and/or expensive BRIEF SUMMARY OF THE INVENTION Such a technique is particularly suitable for achieving a 1 uniform charge density on an insulator surface, such as a layer of polyethylene terephthalate. The conductive liquid should have a sufficiently high vapor pressure to permit the necessary evaporation thereof. By sufiiciently high vapor pressure" as used in this specification and claims appended thereto, it is meant that the liquid will evaporate under ambient conditions without further inducement or it can be caused to evaporate by the gentle application of heat, said heat being insufficient to adversely affect the layer which is being uniformly charged. In either event, the material should not have such a high vapor pressure that it will not uniformly distribute the charge on the surface prior to complete evaporation.

Applicable liquids are chosen based upon the insulator or photoconductive insulator upon which a uniform charge density is to be produced. Other typical liquids include aromatic hydrocarbons, such as benzene and toluene, lower ketones, esters, etc.

The present invention has been and will be further described with reference to an insulating surface. By insulating it is meant that the material will retain surface electrostatic charge for at least a sufficient period of time for subsequent imaging. The term is meant to include materials generally considered to be insulators, such as polyethylene terephthalate film, and photoconductive insulators materials whose electrical conductivity is affected by exposure to light or other radiation, such as selenium layers or pigmented material, such as zinc oxide,'in an appropriate binder, as well as other materials which will function inaccordance with the above definition.

The nature of the invention will :more easily be understood when it is considered in conjunction with the accompanying drawings wherein:

FIGS. 1A to IE are a diagrammatic illustration of the basic flow steps in the process of the present invention.

FIG. 1A shows the electrostatic charging of imaging member 10 comprising a conductive support member 11 having a layer of insulating material 12 thereover. Conductive support member 11 may comprise such materials as aluminum, brass or other metals, metallized paper or paper with a relatively high moisture content, glass with a transparent or other conductive coating, or like known layer. Insulating layer 12 may comprise any of a number of insulating materials known to be useful in image formation including, for example, polyethylene terephthalate film. Imaging member 10 is negatively electrostatically charged by moving relative thereto a corona charging device 13 which isconnected to a high voltage power supply 14. As previously indicated, corona charging devices are well known in the art and suitable ones for use in the practice of the present invention are described, for example, in U.S. Pat. Nos. 2,777,957 and 2,836,725. FIG. 1B

shows the nonuniform charge distribution caused by the negacan be dispensed manually from a test tube 16. Care should be taken so that the conductive liquid does not come in contact with the underlying conductive support member 11. FIG. 1D shows the uniform redistribution of the negative charges prior to liquid evaporation. FIG 1E shows the insulator layer having the uniform charge distribution after the evaporation of the conductive liquid. To speed evaporation, gentle heat can be applied to the underside of member 10, or hot air flowed over the top side, such heat being insufficient to adversely affect layer 12 whether it be photoconductive or nonphotoconductive. Insulator layer can now be subjected to standard imaging techniques. For example, if layer 12 is an insulator, charge can be conducted to ground by contacting the layer in imagewise configuration with a conductive member, A latent electrostatic image will remain. The latent electrostatic image which remains after image formation can be developed into a visible image by standard development techniques.

T Other modifications of this technique of contact redistribution will be apparent to those skilled in the an. Other techniques include, for example, applying the conductive liquid in the form of an aerosol, flowing a conductive plasma 3 over the charged surface, etc., such techniques being fully applicable to redistribute a nonuniform negative charge to give a uniform charge density.

DESCRIPTION OF SPECIFIC EMBODIMENT The following example is given to enable thoseskilled in the art to more clearly understand and practice the invention. It should be considered not as a limitation upon the-scope of the invention but merely as being illustrative thereof.

. A 3 mil polyethylene terephthalate film on an aluminum substrate is negatively corona charged. A layer of ethanol is deposited on the nonuniformly charged surface and, after evaporation thereof, powder cloud development indicates the surface potential to be uniform within 5 volts.

While the invention has been described with reference to the preferred embodiment thereof, it should be understood by those skilled in the art that various changes'in form and details may be made without departing'from the true spirit and scope of the invention. All such modifications, etc. are considered to be within the scope of the present invention as defined by the claims-appended hereto. 4

We claim: I

1.. A methodof charging the surfaceof an insulator to a substantially uniform potential'comp risingthe steps of:

a. charging the surface of an insulator to a potential, said potential being of a nonuniform density; and b. applying a layer of conductive liquid having sufiicient lateral conductivity to said insulator surface subsequent to the charging; thereof, thereby redistributing said surface charge to attaina uniform charge density on said insulator surface.

2. The method of claim I further including the. step of evaporating'said conductive liquid from said insulator surface to provide a uniform charge density after evaporation.

3. The method of claim 1 wherein the insulating surface initially has a nonuniform'negative charge thereon.

4. The method of claim 1 wherein the conductive liquid has a highvapor pressure. 1

5. The-method of claim 1 wherein the conductive liquid is ethanol. 

